1. Field of the Invention
The invention relates generally to miniature electromagnetic switches for microwave communication systems. More specifically, the invention relates to methods of fabricating miniature electromagnetic microwave switches and arrays of miniature electromagnetic microwave switches for coplanar waveguide, transmission lines and microstrip transmission lines.
2. Description of the Prior Art
In a modern microwave telecommunication system, a microwave switch is one of the essential parts. A switch is needed whenever a change of path for a signal or a selection of signals for a transmission line is needed. The basic requirements for such switches are low loss, high speed and small size. The last requirement is especially important for millimeter wave communication systems. The commonly used microwave switches are mostly conventional mechanical switches and semiconductor switches. The conventional mechanical switches are slow, bulky and heavy and consume a lot of power. Therefore, they are not appropriate for applications where the resource budgets (size, weight and power) are tight and for millimeter wave communication system applications even though their power handling capability is large. Furthermore, mechanical switches are discrete devices and are difficult to integrate into a switch array or matrix, which is very useful for signal routing in communication systems. One simple example of such applications is a television set with several satellite dishes. For this case, a switch array or a switch box is needed for the selection of the satellites.
Considerable efforts have been made on the development of microwave semiconductor switches. Although their power handling capability is lower than that for the bulk electromechanical switches, the semiconductor switches are fast, small and can be integrated with other components on a semiconductor substrate. These switches could be a field effect transistor (FET) or a PIN diode. The performance of the semiconductor switches are limited by the finite electrical resistance and capacitance associated with the semiconductor junctions. In the ON state of a semiconductor switch, the finite resistance at the junctions and in the semiconductor itself contribute significantly to the insertion loss. In the OFF state, the relatively large capacitance of the reversed-biased semiconductor junctions usually lead to isolation inferior to mechanical switches.
Although mechanical and semiconductor switches have performance characteristics sufficiently adequate for many applications, microwave switch designers are always on the lookout of better switches--switches with higher microwave performances, higher power, smaller size and higher switching speed. Microelectromechanical (MEM) switches offer the high isolation and smaller insertion loss similar to mechanical switches but with size not much bigger than semiconductor switches. The switching speed of MEM switches lies between mechanical and semiconductor switches. MEM switches based on electrostatic actuation have been invented and demonstrated good switching properties in recent years. These include the rotating switch disclosed in U.S. Pat. No. 5,121,089 granted to L. E. Larson. In his switches, a rotating switchblade rotates about a hub under the influence of an electrostatic field created by control pads on the same substrate. A microwave signal can then be selectively transmitted along the transmission lines. The switches demonstrate excellent impedance match and very small loss. However, the lifetime of these switches is small because of wearing of the turning parts. In U.S. Pat. No. 5,619,061 granted to C. P. Goldsmith, microwave MEM switches with both ohmic and capacitive coupling of the rf lines were described. In these switches, electrostatic force is used to pull a membrane down to connect two microstrip lines. To pull down the membrane, a voltage of several tens of volts must be applied to the controlling electrode. There is the problem of sticking and electric charges accumulation on the dielectric membrane. To overcome these problems, a novel MEM switch, which is based on electromagnetic actuation, suitable for microwave applications has been invented and will be described in this patent.